Introduction

APD is the Advanced PHP Debugger. It was written to provide profiling and
debugging capabilities for PHP code, as well as to provide the ability to
print out a full stack backtrace. APD supports interactive debugging, but
by default it writes data to trace files. It also offers event based
logging so that varying levels of information (including function calls,
arguments passed, timings, etc.) can be turned on or off for individual
scripts.

Caution

APD is a Zend Extension, modifying the way the internals of PHP handle
function calls, and thus may or may not be compatible with other Zend
Extensions (for example Zend Optimizer).